Direct-coupled multimode WDM optical data links with monolithically-integrated multiple-channel VCSEL and photodetector

ABSTRACT

A low cost, high speed WDM optical link which employs a circular array of VCSEL emitters directly coupled to a multi-mode fiber through which multiple simultaneously optical transmissions are sent to an optical detector comprising a circular array of RCPD detectors receiving the multiple simultaneous transmissions. The emitters and detectors within each array are fabricated with a double-absorber design which avoids position sensitivity related to the cavity standing wave and eliminates the need for in-situ cavity-mode adjustment. In addition, a coupled-cavity structure is presented to achieve passbands with flat tops and steep sides to approach the ideal square-shape photoresponse and to ensure proper channel alignment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application Ser.No. 60/105,704 filed on Oct. 26, 1998.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No.DAAH04-95-1-0624, awarded by the Army. The Government has certain rightsin this invention.

REFERENCE TO A MICROFICHE APPENDIX

Not Applicable

INCORPORATION BY REFERENCE

The following publications which are referred to herein using a numberin square brackets (e.g., [1]) are incorporated herein by reference.

[1] S. Y. Hu, J. Ko, E. R. Hegblom, and L. A. Coldren, “Multimode WDMoptical data links with monolithically-integrated multiple-channel VCSELand photodetector arrays,” IEEE J. Quantum Electron., vol. 34, pp.——,August 1998.

[2] S. Y. Hu, J. Ko, and L. A. Coldren, “High-performance densely-packedvertical-cavity photonic integrated emitter arrays for direct-coupledWDM applications,” IEEE Photon. Technol. Leff., vol. 10, pp.766-768,June 1998.

[3] S. Y. Hu, J. Ko, E. R. Hegblom, and L. A. Coldren, “High-performancemultiple-wavelength vertical-cavity photonic integrated emitter arraysfor direct-coupled multimode optical links,” Proc. CLEO'98 conference,San Francisco, Calif., May 3-8, 1998, paper no. CThK1, Invited.

[4] S. Y. Hu, S. Z. Zhang, J. Ko, J. E. Bowers, and L. A. Coldren, “1.5Gb/s/ch operation of multiple-wavelength vertical-cavity photonicintegrated emitter arrays for low-cost WDM local-area networks,”Electron. Lett., vol. 34, No. 8, pp. 768-770, April 1998.

[5] S. Y. Hu, J. Ko, O. Sjolund, and L. A. Coldren, “Optical crosstalkin monolithically-integrated multiple-wavelength vertical-cavity laserarrays for multimode WDM local-area networks,” Electron. Left., vol. 34,No. 7, pp. 676-678, April 1998.

[6] S. Y. Hu, E. R. Hegblom, and L. A. Coldren, “Multiple-wavelengthtop-emitting vertical-cavity photonic integrated emitter arrays fordirect-coupled wavelength-division multiplexing applications” Electron.Lett., vol. 34, No. 2, pp. 189-190, January 1998.

[7] L. A. Coldren, E. R. Hegblom, Y. A. Akulova, J. Ko, E. M.Strzelecka, and S. Y.

Hu, “VCSELs in '98: What we have and what we can expect,” SPIEProceedings, San Jose, Calif., January 28-29, Vol. 3286, 1998.

[8] S. Y. Hu, E. R. Hegblom, and L. A. Coldren, “Multiple-wavelengthtop-emitting vertical-cavity laser arrays for wavelength-divisionmultiplexing applications,” Proc. 10th IEEE LEOS annual meeting, SanFrancisco, Calif., Nov. 10-13, 1997, postdeadline paper PD1-6.

[9] S. Y. Hu, J. Ko, and L. A. Coldren, “Pie-shaped resonant-cavityInGaAs/InAlGaAs/InP photodetector arrays for direct-coupled wavelengthdemultiplexing applications,” Proc. 10th IEEE LEOS annual meeting, SanFrancisco, Calif., November 10-13, 1997, Paper TuJ4.

[10] S. Y. Hu, J. Ko, and L. A. Coldren, “1.55-μm pie-shapedresonant-cavity photodetector arrays for direct-coupled wavelengthdemultiplexing applications,” Electron. Lett., vol. 33, pp. 1486-1488,August 1997.

[11] S. Y. Hu, E. R. Hegblom, and L. A. Coldren, “Coupled-cavityresonant photodetectors for high-performance wavelength demultiplexingapplications,” Appl. Phys. Lett., vol. 71, pp.178-180, July 1997.

[12] S. Y. Hu, J. Ko, and L. A. Coldren, “Resonant-cavityInGaAs/InAlGaAs/lnP photodetector arrays for wavelength demultiplexingapplications,” Appl. Phys. Lett., vol. 70, pp. 2347-2349, May 1997.

[13] E. C. Vail, G. S. Li, W. Yuen, and C. J. Chang-Hasnain, “Highperformance and novel effects of micromechanical tunable vertical-cavitylasers,” IEEE J. Select. Topic Quantum Electron., vol. 3, pp. 691-697,1997.

[14] T. Wipiejewski, J. Ko, B. J. Thibeault, and L. A. Coldren,“Multiple wavelength vertical-cavity laser array employing molecularbeam epitaxial regrowth,” Electron. Lett., vol. 32, pp. 340-342,1996.

[15] D. L. Huffaker and D. G. Deppe, “Multiwavelength, densely-packed2×2 verticalcavity surface-emitting laser array fabricated usingselective oxidation,” IEEE Photon. Technol. Lett., vol. 7, pp. 858-860,1996.

[16] W. Yuen, G. S. Li, and C. J. Chang-Hasnain, “Multiple-wavelengthvertical-cavity surface-emitting laser arrays,” IEEE J. Select. TopicQuantum Electron., vol. 3, pp. 422-428, 1997.

[17] A. Fiore, Y. A. Akulova, E. R. Hegblom, J. Ko, and L. A. Coldren,“postgrowth tuning of cavity resonance for multiple-wavelength laser anddetector arrays” Proc. CLEO'98 conference, San Francisco, Calif., May3-8, 1998, paper no. CThX3.

[18] H. Hasegawa, K. E. Forward, and H. Hartnagel, “Improved method ofanodic oxidation of GaAs”, Electron. Left., Vol.11, pp. 53-54,1975.

[19] K. H. Hahn, M. R. T. Tan, Y. M. Houng, and S. Y. Wang, “Large areamultitransverse-mode VCSELs for modal noise reduction in multimode fibersystems,” Electron. Left., vol. 29, pp.1482-1483, 1993.

[20] C. A. Burrus and B. I. Miller, “Small-area double heterostructureAlGaAs electroluminescent diode sources for optical fiber transmissionlines,” Opt. Commun., vol. 4, pp. 307-309, 1971.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to device structures and simple packagingschemes to realize low-cost, yet high-performance, multimodewavelength-division multiplexing (WDM) optical data links.

2. Description of the Background Art

(a) Introduction

The demand for ever faster data transmission rates (a few Gb/s up to 100Gb/s) has attracted considerable interest in the development ofhigh-capacity optical data links for short-haul local-area networks andfiber-to-the-desktop applications. The majority of work to date hasfocused on one-dimensional parallel optical data links which utilizemultimode fiber ribbons with a one-data-channel-per-fiber arrangement.However, the maximum aggregate data transmission rate is limited toabout 2-3 gigabytes per second and the system configuration is costlyand very complicated.

One definitive solution to the bandwidth problem is to take advantage ofthe extra-wide bandwidth of optical fibers by employing thewavelength-division multiplexing (WDM) configuration which cansignificantly expand the transmission capacity by having multiple datachannels in each fiber. With WDM, however, the corresponding transmitterand receiver modules must be low cost to be attractive for emerging“gigabytes-to-the-desktop” applications. A problem with WDM is that anyadditional complexity in device fabrication and packaging technology candramatically increase the manufacturing cost. The availability of suchlow-cost multiple-wavelength emitter and detector arrays is also a keyissue for the realization of ultra-high-density multiple-layer digitalversatile disk technology. Obviously, the vertical-cavity devicestructure is the ideal candidate for WDM configurations because theresonant wavelength can be easily varied and its fiber packaging ispotentially low-cost.

(b) VCSEL Emitters in Optical Data Links

With the inherent advantages of its two-dimensional configuration,efficient fiber coupling, and on-wafer testing capability, VCSEL(Vertical-Cavity Surface-Emitting Laser) structures have remained thepreferred candidate for free-space interconnects and optical fibercommunication since inception. Within the last decade, there have beennumerous advances in epitaxial technology, device design, and processingtechniques such that the performance of VCSELs has been greatlyimproved. Currently, major accomplishments in VCSEL technology have beendemonstrated in the 0.8-1.00 μm wavelength regime, where VCSELs arebeing incorporated into many advanced optical systems as ahigh-performance and yet low-cost solution for short-distancecommunications.

VCSELs are the ideal laser structures for the implementation ofwavelength-division multiplexing (WDM) systems because the lasingwavelength can be easily varied by adjustment of the cavity length.However, due to the difficulty of achieving convenient andhigh-reflectivity distributed-Bragg reflector (DBR) mirrors in thelongwave-length (1.3-1.55 μm) regime, in-plane distributed feedback(DFB) laser arrays have been the traditional structures in long-haul WDMoptical communications. On the other hand, It is difficult to makein-plane DFB lasers at 0.8-1.00 μm for short-haul opticalcommunications, and VCSEL technology at 0.8-1.00 μm has matured to apoint where it is possible to build VCSEL-based WDM optical data linksfor short-haul optical interconnects.

In recent years, micromechanically tunable VCSELs [13] andmonolithically integrated multiple-wavelength VCSEL arrays [14]-[17]have been reported by various groups for free-space or fibercommunications. Some of these can be incorporated into the presentinvention. In particular, the anodic oxidation scheme [18] or postgrowthtuning scheme [17] can easily provide the required device density andarbitrary wavelength variation within a small region. These also allowone to perform very flexible and accurate cavity-mode adjustment. Thefollowing discussion focuses on the utilization of anodic oxidation inthe production of multiple-wavelength VCSEL PIE arrays.

To illustrate the wavelength adjustment of VCSELs using the anodicoxidation approach, the well-established 0.98 μm VCSEL design withpseudomorphic InGaAs/GaAs quantum wells (QWs) and AlGaAs/GaAs DBRs willnow be discussed.

FIG. 1 provides an example of the aluminum and indium compositionprofiles for a multiple-wavelength bottom-emitting VCSEL [1]-[5]. Thelaser structure consists of a 32-period top p-mirror and a 19.5-periodoutput n-mirror. The compositions for the AlGaAs quarter-wave layers arepure AlAs for the first 18 periods of the n-DBR and Al_(0.9)Ga_(0.1)Asfor the rest of the structure, except for the oxide-aperturing layerwhich is located in the first AlGaAs quarter-wave layer above the activeregion and has a 400 Å Al_(0.98)Ga_(0.02)As layer in the middle forselective lateral oxidation. The active region consists of three 80 ÅInGaAs QWs with 120 Å GaAs barriers in between. To achievemulti-wavelength VCSEL PIE arrays by anodic oxidation requires twoepitaxial growths. The first growth stops at the GaAs phase-tuning layerand then cavity modes for the individual channels are adjusted byperforming anodic oxidation on this phase-tuning layer prior to thegrowth of the rest of the top DBR mirror. In FIG. 1, the phase-tuninglayer is the fourth GaAs layer above the 1-λ cavity, where λ is theBragg wavelength of the DBRs. The effects of choosing different tuninglayer locations are covered in the following discussion.

FIG. 2A shows the corresponding calculated lasing wavelength, while FIG.2B shows the corresponding threshold gain per quantum well as a functionof the location and thickness of the GaAs phase-tuning layer. Thenumber, m, shown beside each curve denotes that the tuning layer is them-th GaAs layer above the 1-λ cavity. For the special case: m=0, thetuning layer is the 1-λ cavity itself. When m>0and if the GaAs tuninglayer is 0.75-λ (point A in the figure), it behaves just like a normalquarter-wave layer and the device will lase at the Bragg wavelength λ.However, when the GaAs tuning layer is 0.5-λ or 1.0-λ (points B and C inthe figure), this layer itself becomes a second cavity in the wholestructure with the same cavity mode as the original 1-λ cavity. As aresult, there will be a splitting in energy levels λ points B and C) dueto the mode-coupling effect. The amount of splitting decreases as thecavity separation increases but the wavelength-tuning curve becomes moreand more nonlinear. Theoretically, the tuning curve will graduallychange from a straight line of slope 2λ/n_(S) ² to a straight line ofzero slope when m increases from 0 to λ. Here n_(S) is the refractiveindex of the cavity. In order to achieve equidistant wavelengthseparation, a small m value, such as m=0 or at most m=1, should be used.Under this circumstance, the freespectral range is wider so that theavailable wavelength span is also larger than the designs with a largerm. However, the overall variation in device threshold currents toachieve this maximum wavelength span will be much greater, asillustrated in FIG. 2B. Moreover, the tolerance in process control ofthe anodic oxidation also becomes tighter because of the steeperwavelength tuning rates. For current short-distance coarse WDMapplications, it is possible to use the m=3 or even m=4 design becausethe channel spacing is still much wider than 4 nm, which is mainlylimited by the receiver bandwidth.

In both FIG. 2A and FIG. 2B, the threshold characteristics of theadjacent higher-order and lower-order modes for the m=4 case (solidlines) are shown to illustrate the mode-coupling effect at the two endsof the free spectral range. This coupling effect caused not onlynonlinearity but also an abrupt switch in device threshold and lasingwavelength. The observed systematic variation in thresholdcharacteristics comes from the extending of optical field intensity fromthe central 1-λ gain cavity into the GaAs phase-tuning layer when thecavity mode is detuned away from the Bragg wavelength of the DBRs.Another factor that also causes reduction in optical confinement factoris the shift of optical standing-wave peak away from the InGaAs QWs.Consequently, threshold current will have its lowest value at the Braggwavelength and goes up in either direction of wavelength. In particular,when the tuning layer thickness is outside the (0.5λ, 1.0λ) range, theadjacent modes have lower threshold gains than the central mode and thuswill dominate the lasing behavior.

The mode splitting at the 1.0-λ tuning layer thickness determines themaximum available wavelength span. FIG. 3 shows the maximum lasingwavelength span and the required maximum threshold gain per well as afunction of m. The solid curves are for the design shown in FIG. 1;while the dotted curves and the experimental data (0) are for thespecial case when we increase the first GaAs layer below the 1-λ cavityto 1.25-λ to avoid exposing AlGaAs layers on the etched surface. Inreality, however, devices will not lase if the required threshold gainis higher than what the active material can provide, especially when thetuning layer thickness is close to 0.5λ or 1.0λ. If the maximum opticalgain is experimentally limited to only 2400 cm⁻¹ from eachIn_(0.2)Ga_(0.8)As quantum well, the solid curve of the maximumwavelength span has to be reduced to the dash-dotted curve by excludingthose wavelength ranges where the required threshold gain is beyond 2400cm⁻¹ per quantum well. The curves suggest that m=2 would be the optimaldesign for providing the widest wavelength-tuning span whosewavelength-tuning characteristics are not overly steep. The latter is animportant consideration for the processing control of anodic oxidation.

The above analysis and designs can be applied to other material systemsat various wavelength regimes as well. The principle issue is thecapability of performing successful epitaxial regrowth over thephase-tuning layer which can be GaAs, AlGaAs, InAlGaAs, InGaAsP, orothers. For example, low-Al-content AlGaAs is very likely to be thetuning layer for 0.85-μm WDM VCSEL PIE arrays so that high-qualityAlGaAsAlGaAs over-growth interface has to be achieved for the abovementioned regrowth method. However, we can also use dielectric DBRmirrors, transverse wet oxidation [17], or other techniques to vary theresonant wavelengths if the regrowth technology is not available.

In addition, modal noise is a well-known phenomenon for multimode fibersystems when a coherent light source is used. For a multimode WDM link,if single-mode VCSELs are used at the transmitter part, output intensityprofile for each VCSEL channel at the other end of the fiber will bevery likely centered in a few spots which are distributed over the 62.5μm-diameter core in an unpredictable manner. Moreover, these spots willmove around the fiber core when the fiber is bent or vibrated somewhere.Therefore, broad bandwidth multimode VCSELs are recommended formultimode WDM systems [19] to produce a uniform intensity profile at thefiber output. In our designs, an oxide aperture extending only 1.0 μmfrom the edges of VCSELs is created to reduce optical losses for thehigh-order modes to facilitate multiple lateral-mode operation.

(c) RCPDs in Optical Data Links

Narrow-band monolithically-integrated RCPD arrays are needed forreal-time spectroscopic analysis or parallel demultiplexing ofwavelength-encoded channels [9]-[12]. The key issue in design is toachieve high quantum-efficiency and narrow-band square-likephotoresponse. Traditionally, there are two major approaches: one byRCPDs and the other by multiple-cavity dielectric filters combined withdiscrete photodetectors. Because of the large numerical apertureassociated with multimode fibers, the RCPD approach is expected to bemuch better than the other to generate narrow-band photoresponse [1].

In view of the foregoing, a need exists for a direct coupled-multimodeWDM data link with monolithically-integrated multiple-channel VCSEL andphotodiodes arrays. The present invention satisfies that need, as wellas others, and overcomes deficiencies in prior approaches.

BRIEF SUMMARY OF THE INVENTION

The present invention is a direct-coupled optical data link structurefor low-cost multimode wavelength-division multiplexing (WDM) inhigh-capacity (a few Gb/s up to 100 Gb/s) local-area network orfiber-to-the-desktop applications. This link comprises novelmonolithically-integrated multiple-channel vertical-cavitysurface-emitting laser (VCSEL) and narrow-band resonant-cavityphotodetector (RCPD) arrays with their emitting or photon-collectingelements closely-packed within a small diameter circular area tofacilitate self-aligned coupling to a single multimode fiber [1]-[12].For multimode fibers, the devices are contained within a 60 μm circle oralternately sized areas for matching to different fiber cores. For otherfibers, this size may be varied to properly inject light from the VCSELelements into the guided fiber modes. Unlike other techniques for makingmultiple wavelength laser or photodetector arrays, the present inventionallows a multiple-channel VCSEL/RCPD link to be packaged in the samemanner as a single-channel data link without resorting to complicatedand expensive waveguide-coupling optics. Furthermore, one may thencombine in parallel several of these direct-coupled WDM links to a fiberribbon cable to thereby multiply the overall system capacity.

In order to achieve high-performance monolithically-integratedmultiple-channel VCSEL and RCPD arrays at a low cost, the inventionemploys a circular device arrangement to obtain uniform and efficientfiber coupling. The individual VCSELs or RCPDs of the array can bedesigned with a conventional circular shape or a unique “pie”-like shapeto maximize the light-emitting or photon-collecting area. Wavelengthadjustment can be performed by several methods, including anodicoxidation and regrowth or postgrowth variable oxidation. The focus ofthese methods is to provide precisely-controlled anodic oxidation with aconvenient binary-coding method. The advantage of this method over priormodified growth wavelength-adjustment techniques is that it can easilyprovide the required device density and arbitrary wavelength variationwithin a small photolithographically-defined region. For theVCSEL-transmitters, a unique and manufacturable planar-surface VCSELstructure is employed to facilitate device packaging. The idea is todefine the VCSEL pillars, i.e. the closelypacked photonic-integratedemitter (PIE) array, by etching a narrow deep trench (about 3-5 μm wide)around the VCSEL pillars and filling up the trench with an insulator,such as polyimide, which facilitates the addition of the metalcross-overs. On the RCPD-receiver side, the simplest broadcast-typeconfiguration is adapted in this link, and wavelength-selectivenarrow-band RCPDs are achieved by minimizing cavity losses andincreasing cavity length, i.e., to maximize the cavity Q-factor. A noveldouble quantum-well absorber design has been used to ensure a uniformphotoresponse over the whole wavelength range covered by the RCPD array.In addition, a novel coupled-cavity structure is proposed to achievepassbands with flat tops and steep sides to ensure proper channelalignment.

An object of the invention is to provide a method whereby low costwavelength division multiplexed (WDM) optical link may be produced.

Another object of the invention is to provide for inexpensivelypackaging the multiple channels of a WDM optical link without the needof waveguide-coupling optics.

Another object of the invention is to provide a method of creatingplanar surface VCSELs which are easy to manufacture.

Another object of the invention is to provide general guidelines for WDMVCSEL array design, whether planar or not.

Further objects and advantages of the invention will be brought out inthe following portions of the specification, wherein the detaileddescription is for the purpose of fully disclosing preferred embodimentsof the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a diagram showing an example of the aluminum and indiumcomposition profiles for a multiple-wavelength bottom-emitting VCSEL.

FIG. 2A and FIG. 2B are graphs of calculated lasing wavelength andthreshold gain, respectively, per quantum well as a function of thelocation and thickness of the GaAs phase-tuning layer for amultiple-wavelength bottom emitting VCSEL.

FIG. 3 is a graph showing the maximum lasing wavelength span and therequired maximum threshold gain per well as a function of m for amultiple-wavelength bottom-emitting VCSEL.

FIG. 4 shows a schematic multimode WDM optical data link according tothe present invention.

FIG. 5 shows the layout of an array of VCSELs or RCPDs having aconventional circular shape.

FIG. 6 show the layout of an array of VCSELs or RCPDs having a “pie”shape.

FIG. 7 is a schematic cross-section of a multiple-wavelengthbottom-emitting VCSEL PIE array with a self-aligned Burrus-typefiber-coupling configuration according to the present invention.

FIG. 8 diagrammatically depicts a finished bottom-emitting VCSEL PIEarray corresponding to FIG. 7.

FIG. 9 is a schematic cross-section of a multiple-wavelengthtop-emitting VCSEL PIE array with a self-aligned Burrus-typefiber-coupling configuration according to the present invention.

FIG. 10 diagrammatically depicts a finished top-emitting VCSEL PIE arraycorresponding to FIG. 9.

FIG. 11 diagrammatically depicts a bottom-emitting VCSEL PIE arrayaccording to the present invention after undergoing the p-contactprocess.

FIG. 12A through FIG. 12C are graphs of the experimental light-currentfor an eight-element bottom-emitting VCSEL PIE array according to thepresent invention.

FIG. 13A through FIG. 13C are graphs of threshold characteristics andlasing spectra of a top-emitting VCSEL PIE array according to thepresent invention.

FIG. 14 is a schematic cross-section of a top-emitting VCSEL structureaccording to the present invention.

FIG. 15 is a graph of bandwidth absorption for a top-emitting VCSELstructure as shown in FIG. 14.

FIG. 16A is a graph of modulation responses as a function of frequencyat various bias levels for the bottom-emitting VCSEL of FIG. 7.

FIG. 16B is a graph of 3dB modulation bandwidth, f_(3dB), as a functionof (I-I_(th))^(½) where I is the bias current and I_(th) is thethreshold current for the bottom-emitting VCSEL of FIG. 7.

FIG. 17 is a schematic cross-section of a high-speed VCSEL PIE designaccording to the present invention.

FIG. 18 is a schematic cross-section of a bottom-illuminatedmultiple-channel resonant-cavity Schottky photodetector array accordingto the present invention.

FIG. 19 is a diagrammatic top view of a finished eight-channel“pie”-shaped photodetector array according to the present invention.

FIG. 20 is a diagram of refractive index profile for abottom-illuminated Schottky structure according to the presentinvention.

FIG. 21 is a diagram of the refractive index profile for a P-i-Nstructure according to the present invention.

FIG. 22 is a schematic cross-section view for a P-i-N photodetectoraccording to the present invention.

FIG. 23 is a graph of theoretical reflectivity spectra for positionsunder the Au back mirror at predetermined tuning layer thicknesses, forthe structure shown in FIG. 22.

FIG. 24 is a graph comparing theoretical cavity-mode tuningcharacteristics for the bottom-illuminated Schottky and P-i-Nphotodetector designs shown in FIG. 18 and FIG. 22, respectively.

FIG. 25 is a diagram of the refractive index profile of a coupled-cavitySchottky photodiode design according to the present invention.

FIG. 26 is a graph of the theoretical reflectivity spectra for thesingle-cavity (both Schottky and P-i-N) and the coupled-cavity(Schottky) structures according to the present invention.

FIG. 27 is a graph of angular variation in relation to optical powerabsorption for incident planar waves for the Schottky design shown inFIG. 18 with a normally-incident resonant wavelength of 978 nm.

FIG. 28 is a graph of the angular variation of the resonant wavelengthusing a resonant-cavity photodetector (solid curve) or a SiO₂/SiN_(x)dielectric filter (dash curve) design according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 4, a schematic diagram of a multimode WDMoptical data link 10 according to the present invention is shown. Inthis diagram, wavelength-encoded optical signals are transmitted from amonolithically-integrated multiplewave-length VCSEL array 12 to achannel-matched wavelength-selective photodetector array 14 via a singlemultimode fiber 16 without using any waveguide combiner or distributor.This is achieved by circularly arranging all array elements within thearea of a fiber core so that the whole array can be simultaneouslycoupled to a single fiber by one alignment procedure. The multimodefiber 16 is shown interfacing with the transmitter array 12 and receiverarray 14 by means of an epoxy bond 18. The individual VCSELs or RCPDs ofthe arrays can be designed with a conventional circular shape as shownin FIG. 5 or the “pie” shape as shown in FIG. 6. Ideally, both laser andphotodetector arrays should have planar-surface structures to facilitatefurther device packaging. Additionally in order that the arrays bemass-produced they must be easy to manufacture. A detailed descriptionof the transmitter and receiver designs follow and additionally asampling of preliminary experimental results is provided.

(a) VCSEL Transmitters

Both bottom-emitting and top-emitting VCSELs can be used for thetransmitter modules via different packaging methods. FIG. 7 shows aschematic cross-section for a multiple-wavelength bottom-emitting VCSELPIE array 40 formed on a substrate 42. A multimode fiber 44 withself-aligned Burrus-type [20] fiber-coupling configuration is attachedto the substrate with epoxy 46. The substrate 42 of this embodiment ispreferably comprised of the following series of layers: an AuGe/Ni/Aulayer 48 for n-contact metallization, a GaAs substrate layer 50, ann-DBR layer 52, an active layer 54, a p-DBR layer 56, an insulatingisolation layer 58, and a layer of Ti/Pt/Au 60 for contacting the p-DBRand for probing pads. Within this structure are shown an insulating,isolation layer 62 of, for example SiN_(x), and polyimide 64 wells andoxide apertures 66. FIG. 8 is a rendering of a scanning-electronmicrograph (SEM) image of a finished bottom-emitting VCSEL PIE arraywhich comprises eight “pie-shaped” VCSELs emitting eight differentwavelengths. This structure corresponds to the configuration shown inFIG. 6. The exact number of elements in each array will be determined bythe specific applications. However, the upper limit is usually set byprocessing techniques and device performance. Additionally, note thatthe “pie” shape is not essential. Other VCSEL shapes such as circles,triangles or squares may also be used as long as they fit within thecircumference of the fiber core to which the VCSEL PIE array is to beattached.

Similarly, FIG. 9 and FIG. 10 illustrate the corresponding top-emittingapproach. FIG. 9 shows a schematic cross-section for a proposedmultiple-wavelength top-emitting VCSEL PIE array 80 formed on asubstrate 82. A multimode fiber 84 is attached to the substrate withepoxy 86. The substrate 82 of this embodiment preferably comprises thefollowing series of layers: an AuGe/Ni/Au layer 88 for n-contactmetallization, a GaAs substrate layer 90, an n-DBR layer 92, an activelayer 94, and a p-DBR 96, an insulating isolation layer 98, with a layerof Ti/Pt/Au 100 for contacting the p-DBR and for probing pads. Withinthis structure are shown a SiN_(x) 102 anti-reflection coating andpolyimide 104 wells along with oxide apertures 106.

FIG. 10 is a rendering of a scanning-electron micrograph (SEM) image ofa finished top-emitting VCSEL PIE array which comprises eight VCSELsemitting eight different wavelengths. This metallization retains theprevious “pie” structure yet employs circular contact areas.

The processing steps for the foregoing VCSEL PIE arrays is as follows.After epitaxial regrowth, the sample is patterned with photoresist foretching of the deep narrow trenches (3-5 μm wide) around the VCSELs fordevice isolation and access to the wet oxidation layer. As a result,arrays of eight pie-shaped VCSEL mesas were created after a pure C12reactive-ion etching. These eight VCSELs are closely packed within a60-μm-diameter circle to match the core of a multimode fiber. In orderto avoid having an exposed Al_(0.9)Gao.IAs layer at the bottom of thetrenches, the first GaAs quarter-wave layer below the active region wasdesigned with a thickness of 1.2 λ and the C12 etching was controlled tostop within this layer. After the trench etching, a short 2.5 minute wetoxidation process at 425° C. is performed to create an oxide apertureextending only 1.0 μm from the edge to facilitate multiple lateral-modeoperation. Then the sample is covered with SiN_(X) and the film on topof the VCSELs is removed by CF₄/O₂ plasma for Au/Zn/Au metallization.

FIG. 11 is a rendering of a scanning-electron micrograph (SEM) image 120of the bottom-emitting VCSEL PIE array after the p-contact process.Before putting down the thick Ti/Pt/Au probing pads, the deep trenches122, 124 are filled with polyimide for metal cross-over and isolation,respectively. The n-contact metallization on the substrate side of thesample is AuGe/Ni/Au. SiN_(x) is used for anti-reflection coating on thebottom-emitting VCSELs. For the Burrus-type packaging scheme, an etchingwell is created in the substrate for self-aligned fiber coupling andalso for better coupling efficiency.

FIG. 12A shows the experimental light-current curves for aneight-element bottom-emitting VCSEL PIE array with the design shown inFIG. 11 (m=4) but also with a 1.25-λ GaAs layer below the 1-λ cavity forthe reason described earlier. FIG. 12B shows the correspondingexperimental and theoretical threshold characteristics. Most deviceslased at around 1.7 mA with a bias voltage of 2.4 V. If we number thedevices according to the tuning layer thickness, channels #1 to #6exhibit similar electrical and spectral characteristics when the outputpower is below 5 mW. Channel #4 has 7.3 mW maximum output power, 50.2%differential quantum efficiency, and 11.8% maximum wall-plug efficiency.As theoretically predicted, the device (channel #8) with a GaAs tuninglayer over 1.0 λ lases at the adjacent higher-order mode. FIG. 12C showsthe lasing spectra of this VCSEL PIE array measured at 5-6 mA. Theachieved lasing wavelength span from this array is 32.9 nm, but themaximum available wavelength span is about 36.7 nm. From FIG. 12B onecan see that only half of the usable tuning range has been used by thissample. It is possible to further optimize the channel allocation sothat all eight channels can be located at the flat part of the thresholdcurve.

Similarly, FIG. 13A through FIG. 13C show the corresponding thresholdcharacteristics and lasing spectra of the top-emitting VCSEL PIE arrays,respectively.

From the system point of view, the bottom-emitting VCSEL PIE scheme ispreferred over the top-emitting version because the bottom-emittingversion provides a larger emitting area for each array element whichwill, in turn, enhance the multimode operation that is recommended formultimode fiber systems [19]. Moreover, the bottom-emittingconfiguration is also compatible with the flip-chip bonding 20 as shownin FIG. 4 which provides adequate heat sinking for reducing thermalcrosstalk between channels. However, the top-emitting VCSEL structurehas the advantage of being able to monolithically integrate withwavelength-matched RCPD arrays, simply by etching off a few periods ofthe top DBR mirror of VCSELs which turns them into reasonably effectivephotodetectors [1], whose structure 140 is illustrated in FIG. 14. Thestructure shows both a dual-core multimode fiber 142 with a receivingfiber 144, and a transmission fiber 146. Both the receiving fiber 144and transmission fiber 146 can be attached to the substrate 148 with itsrespective RCPDs 150 and VCSELs 152 in a single operation. In theembodiment shown, it is desirable to use the transparentindium-tin-oxide (ITO) top contact to avoid the difficulty inphotolithographical alignment. In addition, sometimes it is alsodesirable to adapt the top-emitting scheme to avoid the substantialsubstrate absorption, such as 850 nm wavelength for GaAs substrates.

FIG. 15 gives theoretical design characteristics of the photodetector ofFIG. 14. Both the 3 db optical bandwidth and the fraction of lightabsorbed are shown for a variable number of DBR panels in the topmirror.

Preliminary on-chip high-speed testing of the bottom-emitting VCSEL PIEarrays of the current embodiment, as in FIG. 7, has been performed usinga modified coaxial probe [4]. Laser output was collected by a 62.5μm-core graded-index fiber at the substrate side. FIG. 16A shows thetypical modulation responses at various bias levels measured by a 20 GHzlightwave component analyzer. FIG. 16B shows the 3dB modulationbandwidth, f_(3dB), as a function of (I-I_(th))^(½), where I is the biascurrent and Ith is the threshold current. The observed modulationcurrent efficiency factor is 2.4 GHz/mA¹/², which is similar to that fora conventional 15 μm-diameter etched-post VCSEL which has about the samearea as our “pie”-shape VCSEL (88λμm²). The RC-limited bandwidth of ourdevices is estimated to be around 6.5 GHz, which is close to what wehave obtained. An additional proton implantation under the probing padswill be very helpful to reduce the parasitic capacitance. Datatransmission through a 300m-long graded-index multimode fiber (with a62.5-μm core diameter) has been performed by using an O/E converter asthe receiver. The results show that these devices, designed withoutparticular high-speed consideration, are already capable of transmittingdata up to 1.5 Gb/sec/channel under a 223-1 pseudo-random bit sequence(PRBS) without error (<10-13 bit-error rate). Additional protonimplantation or semi-insulating substrate approach will bring up thehigh-speed modulation performance at the expense of more complicatedprocessing.

FIG. 17 shows an embodiment of a high-speed VCSEL PIE design 180according to the invention. The die 182 contains a multimode fiber 184of the self-aligned Burrus-type [20] fiber-coupling configurationattached to the substrate with epoxy 186. The integrated circuit die 182of this embodiment comprises the following series of layers: an S.I.GaAs substrate layer 188, an i-DBR layer 190, an n-contact layer 192, anactive layer 194, an H+implanted p-DBR layer 196, an insulatingisolation layer 198, and a layer of Ti/Pt/Au 200 for contacting theunimplanted p-DBR and for probing pads. Within this structure are shownwells 202, 204 and 206 filled with polyimide for isolation andplanarization, n-contacts 208, 210 comprising AuGe/Ni/Au, pcontacts 214,216 comprising Pd/Zn/Au, and contact pads 200, 218 comprising Ti/Pt/Au.As it is illustrated, the p-contacts 214, 216 are on top of the VCSELwhile the n-contacts 208, 210 have an intra-cavity configuration.

(b) RCPD receivers

For the direct-coupled WDM receiver designs according to the invention,RCPDs are also made in a circular device arrangement, just like theVCSEL PIE arrays, to facilitate broadcast-type wavelengthdemultiplexing. Therefore, when the optical fiber is aligned andslightly defocused to the center of the pie array, all eight elementswill be equally illuminated with all input wavelengths, but each of themwill only pick up the specific channel which lies within its absorptionband. A splitting loss of 13 dB has been observed in this configurationat 1.55 μm with a single-mode fiber, which is close to the 11 dBtheoretical value [9],[10]. Based on the same technology, we can alsoconstruct a WDM photodetector arrays at 0.98 μm in theInGaAs/AIGaAs/GaAs material system.

FIG. 18 shows a schematic cross-section of a bottom-illuminatedmultiple-channel resonant-cavity Schottky photodetector array 240. Thedie 242 contains a multimode fiber 244 of the self-aligned Burrus-type[20] fiber-coupling configuration attached to the substrate with epoxy246. The integrated circuit die 242 of this embodiment comprises thefollowing series of layers: an AuGeNi layer 247, a GaAs substrate layer248, an AlGaAs/GaAs n-DBR layer 250, a residual absorber layer 252, aSiN_(x) isolation layer 254, a contact pad metallization layer 256, anAuTiPtAu contact metallization 258, an AlGaAs tunable thickness cavity260, and an absorber layer 262. The die structure 242 is attached to asubmount 264 by means of solder pads 266 for electrical contacting andheat sinking.

FIG. 19 is a rendering of a scanning-electron micrograph (SEM) image ofa finished eight-channel “pie”-shaped photodetector array for 0.98-μmWDM optical data links. These devices are narrow-band resonant-cavitySchottky photodiodes with the Schottky Au metal as the back mirror andthe AlGaAs DBR as the front mirror. Cavity modes for the individualdevices can be adjusted by performing anodic oxidation on the i-AIGaAsphase-tuning layers.

Schottky and P-i-N structures are the two common designs forhigh-performance photodiodes in either bottom-illuminating ortop-illuminating configuration, which can be used to create narrow-bandWDM resonant-cavity photodetectors.

FIG. 20 shows the refractive index profile 300 for a bottom-illuminatedSchottky structure having a narrow-band RCPD design. The structurecomprises a p-GaAs substrate 302, a front AlGaAs DBR 304, a GaAs cavity306, a double GaAs QW absorber 308, a phase tuning layer 310, and an Aumirror 312.

FIG. 21 shows the profile 320 for a P-i-N structure with a few mirrorperiods on top to reduce the sensitivity to the tuning layer thickness.The structure comprises a substrate 322, a bottom (front) DBR composedof i-Al_(0.98)Ga_(0.02)As and Al_(0.32)Ga_(0.68)As layers 324, aAl_(0.32)Ga_(0.6) 8As cavity 326, a dual GaAs QW absorber 328, a fewperiods of back mirror 330, a phase tuning layer 332, an SiO₂ isolationlayer 334, and an Au layer 336 to complete the top (back) mirror.

The SiO₂ layer in the P-i-N design is employed to prevent Au spikinginto the lattice during contact annealing and to increase theback-mirror reflectivity. From the processing point of view, theSchottky structure is substantially simpler as the Schottky Au metalitself can be used as the back reflection mirror. However, the interfacebetween the Au Schottky metal and the high Al-content semiconductorlayer is not very stable. Degradation in photoresponse has been observedin previous In_(0.52)Al_(0.48)As/Au devices. Moreover, the photoresponsebandwidth has also been limited by the finite absorption in Au.Therefore, the alternative P-i-N solution becomes much more attractive.A novel P-i-N photodetector design is therefore proposed within thecurrent invention which employs an anti-resonant oxide aperture.

FIG. 22 shows a device schematic for a P-i-N photodetector 360. Thedevice comprises an S.I. GaAs substrate layer 362, a bottom DBR mirror386, an i-GaAs cavity 366, an n⁺-GaAs bottom contact layer to whichmetal contacts 382 and 384 are applied, an InGaAs QW absorber 370, ap-DBR mirror stack 372, a p-GaAs tuning layer 374, a p-contact metal 376enclosing a SiO₂ layer 378 that isolates the contact from the opticalmode, and an Au mirror 380. Underneath the ring p-contact 376, theAl_(0.98)Ga_(o.02)As layers in the top DBR mirror 372 will be oxidizedafter the mesa etching to increase the DBR reflectivity to almost one.As a result, the undesired absorption underneath the ring p-contactwhich may cause line width broadening in the photoresponses can beeliminated over the whole channel span (−30 nm).

FIG. 23 shows the theoretical reflectivity spectra for positions underthe Au back mirror 380 with tuning layer thicknesses of (solid curves,from left to right) 0.40, 0.50, and 0.62λ, respectively. The dashedcurves are for positions under the ring p-contact 376 with the sametuning layer thicknesses. As can be seen, the undesired absorptionoutside the central SiO₂/Au detection region has been significantlysuppressed over the whole wavelength-tuning range of interest. However,the wavelength-tuning behavior for the P-i-N design is not linear, forthe same reason described earlier in the VCSEL section.

FIG. 24 compares theoretical cavity-mode tuning characteristics for thebottom-illuminated Schottky and P-i-N photodetector designs shown inFIG. 18 and FIG. 22. In this calculation, the back DBR mirror for theP-i-N structure is assumed to be 0.5 period (solid curve, m=1) or 1.5periods (dash-dotted curve, m=2) and the SiO₂ layer is 100 nm.

In terms of photoresponse characteristics, it is very desirable to havepassbands with flat tops and steep sides. A novel coupled-cavitystructure has been proposed earlier to implement this requirement [11].The idea is to add a low-loss cavity that is optically coupled to theabsorptive cavity via an intermediate mirror.

FIG. 25 shows the refractive index profile 400 of a coupled-cavitySchottky photodiode design. The coupling of two cavities is desired toflatten the peak and steepen the sides of the photoresponse curve. Thedesign comprises a GaAs substrate 402, a front (bottom) DBR mirror ofGaAs/Al_(0.98)Ga_(0.02)As 404, a front GaAs cavity 406, a centralcoupling mirror of GaAs/Al_(0.92)Ga_(0.08)As 408, a dual InGaAs QWabsorber 410, a back (top) GaAs cavity 412, a phase tuning layer 414,and an Au mirror 416. Preliminary experimental results clearlydemonstrate the expected coupled-cavity effect. Modeling for anInGaAs/GaAs/AlGaAs design reveals that this new coupled-cavity structurewill be able to show a 30-dB channel rejection ratio (in term of opticalpower) even when the optical channel spacing is as narrow as 1 nm. FIG.26 shows a comparison of the theoretical reflectivity spectra for thesingle-cavity (both Schottky and P-i-N) and the coupled-cavity(Schottky) structures, where the absorption coefficient for at InGaAsQWs is assumed to be 1.0 μm⁻¹.

Contrary to the demand for a high numerical aperture in the transmitterdesign to reduce modal noise effects, a large numerical aperture inoptical fiber will significantly deteriorate the photodetectorperformance. FIG. 27 shows the angular variation of optical powerabsorption for incident planar waves of different wavelengths for theSchottky design shown in FIG. 18 with a normally-incident resonantwavelength of 978 nm. It can be seen from the graph, that the apparentresonant wavelength varies with the incident angle. This effect causesthe photoresponse to broaden and thus limits the minimum channelspacing. Nevertheless, the problem is much more severe for theconventional discrete dielectric filter and photodetector approach. FIG.28 shows a comparison of the angular variation of the resonantwavelength by using a resonantcavity photodetector (solid curve) or aSiO₂/SiN_(X) dielectric filter (dash curve) design. The resonant-cavityphotodetector design can have either dielectric or semiconductor DBRs.As one can see, for the 62.5 μm-core fiber with a numerical aperture of0.275, the resonant-cavity photodetector design can allow a muchnarrower channel spacing than the discrete dielectric filter approach.

Accordingly, a novel double-absorber design was described which avoidsposition sensitivity related to the cavity standing wave and eliminatesthe need for in-situ cavity-mode adjustment [12]. The idea is to set thecenter-to-center spacing of the two thin QW absorption layers at λ/4 sothat the field²-absorption integral is independent of the position ofthe absorbers relative to the cavity standing wave. This is especiallyimportant within the disclosed embodiment as the cavity modes in anarray will be varied over a wavelength span of around 30 nm, and it iscrucial to maintain a reasonable balance between mirror reflections andcavity losses over the entire range in order to obtain a uniformphotoresponse in each array. Moreover, in order to reduce crosstalkbetween channels, these devices are designed with a high-Q cavity byminimizing cavity losses and increasing cavity length to obtainnarrow-band photoresponse. In addition, a novel coupled-cavity structureis proposed to achieve passbands with flat tops and steep sides toapproach the ideal square-shape photoresponse and to ensure properchannel alignment [11]. Although the description above contains manyspecificities, these should not be construed as limiting the scope ofthe invention but as merely providing illustrations of some of thepresently preferred embodiments of this invention.

What is claimed is:
 1. An optical data link, comprising: (a) an array ofvertical cavity surface emitting laser (VCSEL) emitters, operating atpredetermined wavelengths, fabricated within a substrate configured forsimultaneous direct coupling of the VCSEL array to an optical fiber; (b)an optical fiber capable of communicating multiple optical wavelengths;and (c) an array of optical detectors coupled to a terminating end ofsaid optical fiber, wherein a set of reception wavelengths generallymatch the wavelengths transmitted by the optical VCSEL emitters suchthat multiple optical wavelengths can be simultaneously communicated athigh-speed from the emitters to the detectors across the optical fiber.2. An optical data link as recited in claim 1, wherein said VCSELemitters are selected from the group consisting of bottom-emitting VCSELemitters and top-emitting VCSEL emitters.
 3. An optical data link asrecited in claim 1, wherein said optical detectors comprise resonantcavity photodetectors having a double-absorber design with afield²-absorption integral that is independent of absorber positionrelative to the cavity standing wave, and wherein each said opticaldetector includes two thin quantum well absorption layers spaced apartat an approximate quarter-wavelength spacing.
 4. An optical data link asrecited in claim 1, wherein the VCSEL array is configured as a circulararray of VCSEL emitters.
 5. An optical data link as recited in claim 1,wherein the VCSEL array is configured with “pie-shaped” optical elementsarranged in a circle.
 6. An optical data link as recited in claim 1,wherein said optical fiber is coupled to said VCSEL array by the use ofa self-aligned Burrus-type fiber-coupling configuration.
 7. An opticaldata link as recited in claim 1, wherein said array of optical detectorscomprises a resonant-cavity photodetector array directly coupled to theoptical fiber.
 8. An optical data link as recited in claim 1, whereinthe VCSEL emitters are wavelength adjustable.
 9. An optical data linkcomprising: (a) an array of vertical cavity surface emitting laser(VCSEL) emitters configured for direct coupling to an optical fiber,wherein the VCSEL emitters operate at various predetermined emissionwavelengths; (b) an array of resonant cavity photodetector (RCPD)detectors configured for direct coupling to an optical fiber, said RCPDdetectors having reception wavelengths generally corresponding to theemission wavelengths of the VCSEL emitters; (c) an optical fiberdirectly coupled between the VCSEL emitter array and RCPD array, whereinmultiple optical wavelengths can be simultaneously communicated athigh-speed from the emitters to the detectors across the optical fiber.10. An optical data link, as recited in claim 9, wherein the VCSELemitters are wavelength adjustable.
 11. A method of creating a multipleband optical link, comprising the steps of: (a) fabricating a verticalcavity surface emitting laser (VCSEL) emitter array on a substrate,wherein the VCSEL emitters are arranged in a circular pattern on thesubstrate and each VCSEL emitter is set for a different emissivewavelength; (b) configuring the substrate with the VCSEL emitters forBurrus-type selfaligning fiber-coupling; (c) fabricating aresonant-cavity photodetector (RCPD) detector array on a substrate,wherein the RCPD detectors are arranged in a circular pattern on thesubstrate and each RCPD is set to detect a different emissivewavelength; (d) configuring the substrate of the RCPD detector array forBurrus-type self-aligning fiber-coupling; (e) bonding the substrates ofthe VCSEL emitter array and RCPD detector array onto electronic circuitsconfigured to receive the respective emitter and detector arrays; and(f) bonding a multi-mode fiber between the VCSEL emitter and RCPDdetector, wherein said bonding is created by means of Burrus-stylecoupling.